Use a dedicated local package in the SPM fixture

The SPM fixture referenced XcodeGen itself as a local package via `../../..`. Its folder reference is named after the resolved directory, so generating fixtures or running tests from a git worktree (e.g. `.claude/worktrees/<name>`) changed the fixture to use the worktree's directory name.

Replace it with a small `Tests/Fixtures/LocalPackage` package that has a library and a test target. This keeps the same coverage (a local package outside the project directory, a custom group, product dependencies and a package test target in the scheme) without depending on the checkout directory name.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>